Surface coatings containing a metallic flake pigment, for example aluminum flake, are well known. They are especially favored for the protection and decoration of automobile bodies, such as for example by reason of their imparting a differential light reflection effect, usually referred, to as “flop”, as well as flake appearance effects, which include flake size distribution and the sparkle imparted by the flake as well as the enhancement of depth perception in the coating. The flop effect is dependent upon the angle from which the car body is viewed. The degree of the flop effect achieved, is a function of the orientation of the metallic flakes with respect to the outer surface of the coating. To attain a maximum flop effect, ideally, the flakes should all lie in planes parallel to this surface. However, in practice it is not possible to obtain more than a proportion of the flakes lying truly parallel, the remainder lie at various angles to the surface plane, i.e. there is a distribution of the orientations of the metallic flakes in the coating. The degree of sparkle is a function of the flake size, surface smoothness, orientation, and uniformity of the edges. Metallic coatings usually also contain pigments, generally of a light absorbing rather than a light scattering type. Any light scatter from the pigments or the flakes themselves, e.g., from the flake edges, diminishes both the flop and the sparkle of the coating.
Instrumental characterization of metallic pigmented coatings can, in principle, be carried out by measuring with a spectrophotometer the spectral reflectance of a coated panel at a number of angles of incident illumination and of viewing, either within the plane of the illumination and viewing axes, or outside of this plane. The results of such measurements are dependent on the degree of flake alignment as well as the type of flake or other pigments used, but give no direct evidence of the degree of sparkle or flake size. As a result, their value in characterizing the coating is insufficient. Additionally, since these measurements are also dependent on the relative concentrations of the metallic flake and on the presence or absence of any light-absorbing or scattering pigment in coating composition, their value in characterizing the coating is diminished. In color matching for example a previously coated substrate of an automotive body, it is necessary to choose the correct pigments to match the color of that substrate as well as the correct flake to match the color and appearance of that substrate. For an effective measure of the flake characteristics such as size or degree of sparkle of the metallic flakes to be obtained, therefore, it is necessary under these circumstances for shaders to select, based on their expertise, the metallic flake to be used by visually analyzing the target surface, such as a previously coated substrate of an automotive body. Once the flake has been identified, the pigments are selected, typically by well known computer based algorithms, such as those based on radiative transfer theory, which mathematically adjust the pigment quantities, add or reduce black and white pigment quantities, and flop adjuster quantities, including flake quantities, so that the error in the color and flop match to the target surface is the lowest while ensuring that the resulting color/flop formulation is still within the bounds of accepted commercial practice. This formulation is then made up, sprayed on test panels, which are then visually compared to the target surface. If the flop and/or sparkle match are deemed unsatisfactory, the shader adjusts the type and/or changes the amount of the metallic flakes entered into the algorithm to get new color/flop formulation and the whole cycle is repeated until an adequate match is achieved in both color and appearance at all angles of illumination and view. The present invention is aimed at a method that substantially reduces the number of repeat matches needed for the selection of metallic flakes that closely match the appearance of metallic flakes present in the target surface.